Network Working Group A. Li, Ed.
Request for Comments: 5109 December 2007
Obsoletes: 2733, 3009
Category: Standards Track
RTP Payload Format for Generic Forward Error Correction
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies a payload format for generic Forward Error
Correction (FEC) for media data encapsulated in RTP. It is based on
the exclusive-or (parity) operation. The payload format described in
this document allows end systems to apply protection using various
protection lengths and levels, in addition to using various
protection group sizes to adapt to different media and channel
characteristics. It enables complete recovery of the protected
packets or partial recovery of the critical parts of the payload
depending on the packet loss situation. This scheme is completely
compatible with non-FEC-capable hosts, so the receivers in a
multicast group that do not implement FEC can still work by simply
ignoring the protection data. This specification obsoletes RFC 2733
and RFC 3009. The FEC specified in this document is not backward
compatible with RFC 2733 and RFC 3009.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
protect against packet loss over packet-switched networks
[9, 10]. In particular, the use of traditional error correcting
codes, such as parity, Reed-Solomon, and Hamming codes, has seen much
application. To apply these mechanisms, protocol support is
required. RFC 2733 [9] and RFC 3009 [11] defined one of such FEC
protocols. However, in these two RFCs a few fields (the P, X, and CC
fields) in the RTP header are specified in ways that are not
consistent as they are designed in RTP [1]. This prevents the
payload-independent validity check of the RTP packets.
This document extends the FEC defined in RFC 2733 and RFC 3009 to
include unequal error protection on the payload data. It specifies a
general algorithm with the two previous RFCs as its special cases.
This specification also fixes the above-mentioned inconsistency with
RFC 2733 and RFC 3009, and will obsolete those two previous RFCs.
Please note that the payload specified in this document is not
backward compatible with RFC 2733 and RFC 3009. Because the payload
specified in this document is signaled by different MIMEs from those
of RFC 3009, there is no concern of misidentification of different
parity FEC versions in capacity exchange. For parity FECs specified
here and in RFC 2733 and RFC 3009, the payload data are unaltered and
additional FEC data are sent along to protect the payload data.
Hence, the communication of the payload data would flow without
problem between hosts of different parity FEC versions and hosts that
did not implement parity FEC. The receiving hosts with incompatible
FEC from the sending host would not be able to benefit from the
additional FEC data, so it is recommended that existing host
implementing RFC 2733 and RFC 3009 should be updated to follow this
specification when possible.
This document defines a payload format for RTP [1] that allows for
generic forward error correction of real-time media. In this
context, generic means that the FEC protocol is (1) independent of
the nature of the media being protected, be it audio, video, or
otherwise; (2) flexible enough to support a wide variety of FEC
configurations; (3) designed for adaptivity so that the FEC technique
can be modified easily without out-of-band signaling; and (4)
supportive of a number of different mechanisms for transporting the
FEC packets.
Furthermore, in many scenarios the bandwidth of the network
connections is a very limited resource. On the other hand, most of
the traditional FEC schemes are not designed for optimal utilization
of the limited bandwidth resource. An often used improvement is
unequal error protection that provides different levels of protection
for different parts of the data stream, which vary in importance.
The unequal error protection schemes can usually make more efficient
use of bandwidth to provide better overall protection of the data
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RFC 5109 RTP Payload Format for Generic FEC December 2007
stream against the loss. Proper protocol support is essential for
realizing these unequal error protection mechanisms. The application
of most of the unequal error protection schemes requires having the
knowledge of the importance for different parts of the data stream.
For that reason, most of such schemes are designed for particular
types of media according to the structure of the media protected, and
as a result, are not generic.
The FEC algorithm and protocol are defined in this document for
generic forward error correction with unequal error protection for
real-time media. The particular algorithm defined here is called the
Uneven Level Protection (ULP). The payload data are protected by one
or more protection levels. Lower protection levels can provide
greater protection by using smaller group sizes (compared to higher
protection levels) for generating the FEC packet. As we will discuss
below, audio/video applications would generally benefit from unequal
error protection schemes that give more protection to the beginning
part of each packet such as ULP. The data that are closer to the
beginning of the packet are in general more important and tend to
carry more information than the data farther behind in the packet.
It is well known that in many multimedia streams the more important
parts of the data are always at the beginning of the data packet.
This is the common practice in codec design since the beginning of
the packet is closer to the re-synchronization marker at the header
and thus is more likely to be correctly decoded. In addition, almost
all media formats have the frame headers at the beginning of the
packet, which is the most vital part of the packet.
For video streams, most modern formats have optional data
partitioning modes to improve error resilience in which the video
macroblock header data, motion vector data, and Discrete Cosine
Transform (DCT) coefficient data are separated into their individual
partitions. For example, in ITU-T H.263 version 3, there is the
optional data partitioned syntax of Annex V. In MPEG-4 Visual Simple
Profile, there is the optional data partitioning mode. When these
modes are enabled, the video macroblock (MB) header and motion vector
partitions (which are much more important to the quality of the video
reconstruction) are transmitted in the partition(s) at the beginning
of the video packet while residue DCT coefficient partitions (which
are less important) are transmitted in the partition close to the end
of the packet. Because the data is arranged in descending order of
importance, it would be beneficial to provide more protection to the
beginning part of the packet in transmission.
For audio streams, the bitstreams generated by many of the new audio
codecs also contain data with different classes of importance. These
different classes are then transmitted in order of descending
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RFC 5109 RTP Payload Format for Generic FEC December 2007
importance. Applying more protection to the beginning of the packet
would also be beneficial in these cases. Even for uniform-
significance audio streams, various time shifting and stretching
techniques can be applied to the partially recovered audio data
packets.
Audio/video applications would generally benefit from the FEC
algorithms specified in this document. With ULP, the efficiency of
the protection of the media payload can potentially be further
improved. This document specifies the protocol and algorithm for
applying the generic FEC to the RTP media payloads.
2. Terminology
The following terms are used throughout this document:
Media Payload: The raw, unprotected user data that are transmitted
from the sender. The media payload is placed inside of an RTP
packet.
Media Header: The RTP header for the packet containing the media
payload.
Media Packet: The combination of a media payload and media header is
called a media packet.
FEC Packet: The FEC algorithms at the transmitter take the media
packets as an input. They output both the media packets that they
are passed, and newly generated packets called FEC packets, which
contain redundant media data used for error correction. The FEC
packets are formatted according to the rules specified in this
document.
FEC Header: The header information contained in an FEC packet.
FEC Level Header: The header information contained in an FEC packet
for each level.
FEC Payload: The payload of an FEC packet. It may be divided into
multiple levels.
Associated: A FEC packet is said to be "associated" with one or more
media packets (or vice versa) when those media packets are used to
generate the FEC packet (by use of the exclusive-or operation). It
refers to only those packets used to generate the level 0 FEC
payload, if not explicitly stated otherwise.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
3. Basic Operation
The payload format described here is used when the sender in an RTP
session would like to protect the media stream it is sending with
generic parity FEC. The FEC supported by this format is based on
simple exclusive-or (XOR) parities operation. The sender takes the
packets from the media stream requiring protection and determines the
protection levels for these packets and the protection length for
each level. The data are grouped together as described below in
Section 7. The XOR operation is applied across the payload to
generate the FEC information. The results following the procedures
defined here are RTP packets containing FEC information. These
packets can be used at the receiver to recover the packets or parts
of the packets used to generate the FEC information.
The payload format for FEC contains information that allows the
sender to tell the receiver exactly which media packets are protected
by the FEC packet, and the protection levels and lengths for each of
the levels. Specifically, each FEC packet contains an offset mask
m(k) for each protection level k. If the bit i in the mask m(k) is
set to 1, then media packet number N + i is protected by this FEC
packet at level k. N is called the sequence number base, and is sent
in the FEC packet as well. The amount of data that is protected at
level k is indicated by L(k), which is also sent in the FEC packet.
The protection length, offset mask, payload type, and sequence number
base fully identify the parity code applied to generate the FEC
packet with little overhead. A set of rules is described in Section7.4 that defines how the mask should be set for different protection
levels, with examples in Section 10.
This document also describes procedures on transmitting all the
protection operation parameters in-band. This allows the sender
great flexibility; the sender can adapt the protection to current
network conditions and be certain the receivers can still make use of
the FEC for recovery.
At the receiver, both the FEC and original media are received. If no
media packets are lost, the FEC packets can be ignored. In the event
of a loss, the FEC packets can be combined with other received media
to recover all or part of the missing media packets.
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RFC 5109 RTP Payload Format for Generic FEC December 20074. Parity Codes
For brevity, we define the function f(x,y,..) to be the XOR (parity)
operator applied to the data blocks x,y,... The output of this
function is another block, called the parity block. For simplicity,
we assume here that the parity block is computed as the bitwise XOR
of the input blocks. The exact procedure is specified in Section 8.
Protection of data blocks using parity codes is accomplished by
generating one or more parity blocks over a group of data blocks. To
be most effective, the parity blocks must be generated by linearly
independent combinations of data blocks. The particular combination
is called a parity code. The payload format uses XOR parity codes.
For example, consider a parity code that generates a single parity
block over two data blocks. If the original media packets are
a,b,c,d, the packets generated by the sender are:
a b c d <-- media stream
f(a,b) f(c,d) <-- FEC stream
where time increases to the right. In this example, the error
correction scheme (we use the terms scheme and code interchangeably)
introduces a 50% overhead. But if b is lost, a and f(a,b) can be
used to recover b.
It may be useful to point out that there are many other types of
forward error correction codes that can also be used to protect the
payload besides the XOR parity code. One notable example is Reed-
Solomon code, and there are many others [12]. However, XOR parity
code is used here because of its effectiveness and simplicity in both
protocol design and implementation. This is particularly important
for implementation in nodes with limited resources.
5. Uneven Level Protection (ULP)
As we can see from the simple example above, the protection on the
data depends on the size of the group. In the above example, the
group size is 2. So if any one of the three packets (two payload
packets and one FEC packet) is lost, the original payload data can
still be recovered.
In general, the FEC protection operation is a trade-off between the
bandwidth and the protection strength. The more FEC packets that are
generated as a fraction of the source media packets, the stronger the
protection against loss but the greater the bandwidth consumed by the
combined stream.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
As is the common case in most of the media payload, not all the parts
of the packets are of the same importance. Using this property, one
can potentially achieve more efficient use of the channel bandwidth
using unequal error protection, i.e., applying different protection
for different parts of the packet. More bandwidth is spent on
protecting the more important parts, while less bandwidth on the less
important parts.
The packets are separated into sections of decreasing importance, and
protection of different strength is applied to each portion - the
sections are known as "levels". The protection operation is applied
independently at each level. A single FEC packet can carry parity
data for multiple levels. This algorithm is called uneven level
protection, or ULP.
The protection of ULP is illustrated in Figure 1 below. In this
example, two ULP FEC packets are protecting four payload packets.
ULP FEC packet #1 has only one level, which protects packets A and B.
Instead of applying parity operation to the entire packets of A and
B, it only protects a length of data of both packets. The length,
which can be chosen and changed dynamically during a session, is
called the protection length.
ULP FEC packet #2 has two protection levels. The level 0 protection
is the same as for ULP FEC packet #1 except that it is operating on
packets C and D. The level 1 protection is using parity operation
applied on data from packets A, B, C, and D. Note that level 1
protection operates on a different set of packets from level 0 and
has a different protection length from level 0, so are any other
levels. Information is all conveyed in-band through the protocols
specified in this document.
Packet A #####################
: :
Packet B ############### :
: :
ULP FEC Packet #1 @@@@@@@@ :
: :
Packet C ########### :
: :
Packet D ###################################
: :
ULP FEC Packet #2 @@@@@@@@@@@@@@@@@
: : :
:<-L0->:<--L1-->:
Figure 1: Unequal Level Protection
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RFC 5109 RTP Payload Format for Generic FEC December 2007
As we have discussed in the introduction, media streams usually have
the more important parts at the beginning of the packet. It is
usually useful to have the stronger protection in the levels closer
to the beginning of the packet, and weaker protection in the levels
farther back. ULP algorithm provides such FEC protection.
ULP FEC not only provides more protection to the beginning of the
packet (which is more important), it also avoids as much as possible
the less efficient scenarios that an earlier section of a packet is
unrecoverable while a later section can be recovered (and often has
to be discarded).
6. RTP Media Packet Structure
The formatting of the media packets is unaffected by FEC. If the FEC
is sent as a separate stream, the media packets are sent as if there
was no FEC.
This approach has the advantage that media packets can be interpreted
by receivers that do not support FEC. This compatibility with
non-FEC capable receivers is particularly useful in the multicast
scenarios. The overhead for using the FEC scheme is only present in
FEC packets, and can be easily monitored and adjusted by tracking the
amount of FEC in use.
7. FEC Packet Structure7.1. Packet Structure
A FEC packet is constructed by placing an FEC header and one or more
levels of FEC header and payload into the RTP payload, as shown in
Figure 2:
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RFC 5109 RTP Payload Format for Generic FEC December 2007
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header (12 octets or more) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Header (10 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Level 0 Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Level 0 Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Level 1 Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Level 1 Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cont. |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: FEC Packet Structure
7.2. RTP Header for FEC Packets
The RTP header for FEC packets is only used when the FEC are sent in
a separate stream from the protected payload stream (as defined in
Section 14). Hence, much of the discussion below applies only to
that scenario. All the fields in the RTP header of FEC packets are
used according to RFC 3550 [1], with some of them further clarified
below.
Marker: This field is not used for this payload type, and SHALL be
set to 0.
Synchronization Source (SSRC): The SSRC value SHALL be the same as
the SSRC value of the media stream it protects.
Sequence Number (SN): The sequence number has the standard definition
- it MUST be one higher than the sequence number in the previously
transmitted FEC packet.
Timestamp (TS): The timestamp MUST be set to the value of the media
RTP clock at the instant the FEC packet is transmitted. Thus, the TS
value in FEC packets is always monotonically increasing.
Payload type: The payload type for the FEC packets is determined
through dynamic, out-of-band means. According to RFC 3550 [1], RTP
participants that cannot recognize a payload type must discard it.
This provides backward compatibility. The FEC mechanisms can then be
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RFC 5109 RTP Payload Format for Generic FEC December 2007
used in a multicast group with mixed FEC-capable and FEC-incapable
receivers, particularly when the FEC protection is sent as redundant
encoding (see Section 14). In such cases, the FEC protection will
have a payload type that is not recognized by the FEC-incapable
receivers, and will thus be disregarded.
7.3. FEC Header for FEC Packets
The FEC header is 10 octets. The format of the header is shown in
Figure 3 and consists of extension flag (E bit), long-mask flag (L
bit), P recovery field, X recovery field, CC recovery field, M
recovery field, PT recovery field, SN base field, TS recovery field,
and length recovery field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|L|P|X| CC |M| PT recovery | SN base |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: FEC Header Format
The E bit is the extension flag reserved to indicate any future
extension to this specification. It SHALL be set to 0, and SHOULD be
ignored by the receiver.
The L bit indicates whether the long mask is used. When the L bit is
not set, the mask is 16 bits long. When the L bit is set, the mask
is then 48 bits long.
The P recovery field, the X recovery field, the CC recovery field,
the M recovery field, and the PT recovery field are obtained via the
protection operation applied to the corresponding P, X, CC, M, and PT
values from the RTP header of the media packets associated with the
FEC packet.
The SN base field MUST be set to the lowest sequence number, taking
wrap around into account, of those media packets protected by FEC (at
all levels). This allows for the FEC operation to extend over any
string of at most 16 packets when the L field is set to 0, or 48
packets when the L field is set to 1, and so on.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
The TS recovery field is computed via the protection operation
applied to the timestamps of the media packets associated with this
FEC packet. This allows the timestamp to be completely recovered.
The length recovery field is used to determine the length of any
recovered packets. It is computed via the protection operation
applied to the unsigned network-ordered 16-bit representation of the
sums of the lengths (in bytes) of the media payload, CSRC list,
extension and padding of each of the media packets associated with
this FEC packet (in other words, the CSRC list, RTP extension, and
padding of the media payload packets, if present, are "counted" as
part of the payload). This allows the FEC procedure to be applied
even when the lengths of the protected media packets are not
identical. For example, assume that an FEC packet is being generated
by xor'ing two media packets together. The length of the payload of
two media packets is 3 (0b011) and 5 (0b101) bytes, respectively.
The length recovery field is then encoded as 0b011 xor 0b101 = 0b110.
7.4. FEC Level Header for FEC Packets
The FEC level header is 4 or 8 octets (depending on the L bit in the
FEC header). The formats of the headers are shown in Figure 4.
The FEC level headers consist of a protection length field and a mask
field. The protection length field is 16 bits long. The mask field
is 16 bits long (when the L bit is not set) or 48 bits long (when the
L bit is set).
The mask field in the FEC level header indicates which packets are
associated with the FEC packet at the current level. It is either 16
or 48 bits depending on the value of the L bit. If bit i in the mask
is set to 1, then the media packet with sequence number N + i is
associated with this FEC packet, where N is the SN Base field in the
FEC packet header. The most significant bit of the mask corresponds
to i=0, and the least significant to i=15 when the L bit is set to 0,
or i=47 when the L bit is set to 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protection Length | mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| mask cont. (present only when L = 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ULP Level Header Format
The setting of the mask field shall follow the following rules:
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RFC 5109 RTP Payload Format for Generic FEC December 2007
a. A media packet SHALL be protected only once at each protection
level higher than level 0. A media packet MAY be protected more
than once at level 0 by different packets, providing the
protection lengths of level 0 of these packets are equal.
b. For a media packet to be protected at level p, it MUST also be
protected at level p-1 in any FEC packets. Please note that the
protection level p for a media packet can be in an FEC packet that
is different from the one that contains protection level p-1 for
the same media packet.
c. If a ULP FEC packet contains protection at level p, it MUST also
contain protection at level p-1. Note that the combination of
payload packets that are protected in level p may be different
from those of level p-1.
The rationale for rule (a) is that multiple protection increases the
complexity of the recovery implementation. At higher levels, the
multiple protection offers diminishing benefit, so its application is
restricted to level 0 for simpler implementation. The rationale for
rule (b) is that the protection offset (for each associated packet)
is not explicitly signaled in the protocol. With this restriction,
the offset can be easily deducted from protection lengths of the
levels. The rationale of rule (c) is that the level of protection is
not explicitly indicated. This rule is set to implicitly specify the
levels.
One example of the protection combinations is illustrated in Figure 5
below. It is the same example as shown in Figure 1. This same
example is also shown in more detail in Section 10.2 to illustrate
how the fields in the headers are set.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Packet A #####################
: :
Packet B ############### :
: :
ULP FEC Packet #1 @@@@@@@@ :
: :
Packet C ########### :
: :
Packet D ###################################
: :
ULP FEC Packet #2 @@@@@@@@@@@@@@@@@
: : :
:<-L0->:<--L1-->:
Payload packet # | ULP FEC packet that protects at level
| L0 L1
---------------------+---------------------------------------
A | #1 #2
B | #1 #2
C | #2 #2
D | #2 #2
Figure 5: An Example of Protection Combination
In this example, ULP FEC packet #1 only has protection level 0. ULP
FEC packet #2 has protection levels 0 and 1. Read across the table,
it is shown that payload packet A is protected by ULP FEC packet #1
at level 0, by ULP FEC packet #2 at level 1, and so on. Also, it can
be easily seen from the table that ULP FEC packet #2 protects at
level 0 payload packets C and D, at level 1 payload packets A-D, and
so on. For additional examples with more details, please refer to
Section 10, "Examples".
The payload of the ULP FEC packets of each level is the protection
operation (XOR) applied to the media payload and padding of the media
packets associated with the ULP FEC packet at that level. Details
are described in Section 8 on the protection operation.
The size of the ULP FEC packets is determined by the protection
lengths chosen for the protection operation. In the above example,
ULP FEC packet #1 has length L0 (plus the header overhead). ULP FEC
packet #2 with two levels has length L0+L1 (plus the header
overhead). It is longer than some of the packets it protects
(packets B and C in this example), and is shorter than some of the
packets it protects (packets A and D in this example).
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Note that it's possible for the FEC packet (non-ULP and ULP) to be
larger than the longest media packets it protects because of the
overhead from the headers and/or if a large protection length is
chosen for ULP. This could cause difficulties if this results in the
FEC packet exceeding the Maximum Transmission Unit size for the path
along which it is sent.
8. Protection Operation
FEC packets are formed from an "FEC bit string" that is generated
from the data of the protected media RTP packets. More specifically,
the FEC bit string is the bitwise exclusive OR of the "protected bit
strings" of the protected media RTP packets.
The following procedure MAY be followed for the protection operation.
Other procedures MAY be used, but the end result MUST be identical to
the one described here.
8.1. Generation of the FEC Header
In the case of the FEC header, the protected bit strings (80 bits in
length) are generated for each media packet to be protected at FEC
level 0. It is formed by concatenating the following fields together
in the order specified:
o The first 64 bits of the RTP header (64 bits)
o Unsigned network-ordered 16-bit representation of the media
packet length in bytes minus 12 (for the fixed RTP header),
i.e., the sum of the lengths of all the following if present:
the CSRC list, extension header, RTP payload, and RTP padding
(16 bits)
After the FEC bit string is formed by applying parity operation on
the protected bit strings, the FEC header is generated from the FEC
bit string as follows:
The first (most significant) 2 bits in the FEC bit string are
skipped. The next bit in the FEC bit string is written into the P
recovery bit of the FEC header in the FEC packet. The next bit in
the FEC bit string is written into the E recovery bit of the FEC
header. The next 4 bits of the FEC bit string are written into the
CC recovery field of the FEC header. The next bit is written into
the M recovery bit of the FEC header. The next 7 bits of the FEC bit
string are written into the PT recovery field in the FEC header. The
next 16 bits are skipped. The next 32 bits of the FEC bit string are
written into the TS recovery field in the FEC header. The next 16
bits are written into the length recovery field in the packet header.
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RFC 5109 RTP Payload Format for Generic FEC December 20078.2. Generation of the FEC Payload
For generation of the FEC payload, the protected bit strings are
simply the protected RTP packets. The FEC bit string is thus the
bitwise exclusive OR of these protected media RTP packets. Such FEC
bit strings need to be generated for each level, as the group of
protected payload packets may be different for each level. If the
lengths of the protected RTP packets are not equal, each shorter
packet MUST be padded to the length of the longest packet by adding
octet 0 at the end.
For protection level n (n = 0, 1, ...), only Ln octets of data are
set as the FEC level n payload data after the level n ULP header.
The data is the Ln octets of data starting with the (Sn + 13)th octet
in the FEC bit string, where:
Sn = sum(Li : 0 <= i < n).
Li is the protection length of level i, and S0 is defined to be 0.
The reason for omitting the first 12 octets is that that information
is protected by the FEC header already.
9. Recovery Procedures
The FEC packets allow end systems to recover from the loss of media
packets. This section describes the procedure for performing this
recovery.
Recovery requires two distinct operations. The first determines
which packets (media and FEC) must be combined in order to recover a
missing packet. Once this is done, the second step is to actually
reconstruct the data. The second step MUST be performed as described
below. The first step MAY be based on any algorithm chosen by the
implementer. Different algorithms result in a trade-off between
complexity and the ability to recover missing packets, if possible.
The lost payload packets may be recovered in full or in parts
depending on the data-loss situation due to the nature of unequal
error protection (when it is used). The partial recovery of the
packet can be detected by checking the recovery length of the packet
retrieved from the FEC header against the actual length of the
recovered payload data.
9.1. Reconstruction of the RTP Header
Let T be the list of packets (FEC and media) that can be combined to
recover some media packet xi at level 0. The procedure is as
follows:
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RFC 5109 RTP Payload Format for Generic FEC December 2007
1. For the media packets in T, compute the first 80 bits of the
protected bit string following the procedure as described for
generating the FEC header in the previous section.
2. For the FEC packet in T, the FEC bit string is the 80-bit FEC
header.
3. Calculate the recovery bit string as the bitwise exclusive OR
of the protected bit string generated from all the media
packets in T and the FEC bit string generated from all the FEC
packets in T.
4. Create a new packet with the standard 12-byte RTP header and
no payload.
5. Set the version of the new packet to 2. Skip the first 2 bits
in the recovery bit string.
6. Set the Padding bit in the new packet to the next bit in the
recovery bit string.
7. Set the Extension bit in the new packet to the next bit in the
recovery bit string.
8. Set the CC field to the next 4 bits in the recovery bit
string.
9. Set the marker bit in the new packet to the next bit in the
recovery bit string.
10. Set the payload type in the new packet to the next 7 bits in
the recovery bit string.
11. Set the SN field in the new packet to xi. Skip the next 16
bits in the recovery bit string.
12. Set the TS field in the new packet to the next 32 bits in the
recovery bit string.
13. Take the next 16 bits of the recovery bit string. Whatever
unsigned integer this represents (assuming network-order),
take that many bytes from the recovery bit string and append
them to the new packet. This represents the CSRC list,
extension, payload, and the padding of the RTP payload.
14. Set the SSRC of the new packet to the SSRC of the media stream
it's protecting, i.e., the SSRC of the media stream to which
the FEC stream is associated.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
This procedure will recover the header of an RTP packet up to the
SSRC field.
9.2. Reconstruction of the RTP Payload
Let T be the list of packets (FEC and media) that can be combined to
recover some media packet xi at a certain protection level. The
procedure is as follows:
1. Assume that we are reconstructing the data for level n, the
first step is to get the protection length of level n (Ln)
from the ULP header of level n.
2. For the FEC packets in T, the FEC bit string of level n is FEC
level n payload, i.e., the Ln octets of data following the ULP
header of level n.
3. For the media packets in T, the protected bit string of level
n is Ln octets of data starting with the (Sn + 13)th octet of
the packet. Sn is the same as defined in Section 8.2. Note
that the protection of level 0 starts from the 13th octet of
the media packet after the SSRC field. The information of the
first 12 octets are protected by the FEC header.
4. If any of the protected bit strings of level n generated from
the media packets are shorter than the protection length of
the current level, pad them to that length. The padding of
octet 0 MUST be added at the end of the bit string.
5. Calculate the recovery bit string as the bitwise exclusive OR
of the protected bit string of level n generated from all the
media packets in T and the FEC bit string of level n generated
from all the FEC packets in T.
6. The recovery bit string of the current protection level as
generated above is combined through concatenation with the
recovery bit string of all the other levels to form the (fully
or partially) recovered media packet. Note that the recovery
bit string of each protection level MUST be placed at the
correct location in the recovered media packet for that level
based on protection length settings.
7. The total length of the recovered media packet is recovered
from the recovery operation at protection level 0 of the
recovered media packet. This information can be used to check
if the complete recovery operation (of all levels) has
recovered the packet to its full length.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
The data protected at the lower protection level is recoverable in a
majority of the cases if the higher-level protected data is
recoverable. This procedure (together with the procedure for the
lower protection levels) will usually recover both the header and
payload of an RTP packet up to the protection length of the current
level.
10. Examples
In the first two examples considered below (Sections 10.1 and 10.2),
we assume that the FEC streams are sent through a separate RTP
session as described in Section 14.1. For these examples, we assume
that four media packets are to be sent, A, B, C, and D, from SSRC 2.
Their sequence numbers are 8, 9, 10, and 11, respectively, and have
timestamps of 3, 5, 7, and 9, respectively. Packets A and C use
payload type 11, and packets B and D use payload type 18. Packet A
has 200 bytes of payload, packet B 140, packet C 100, and packet D
340. Packets A and C have their marker bit set.
The third example (Section 10.3) is to illustrate when the FEC data
is sent as redundant data with the payload packets.
10.1. An Example Offers Similar Protection as RFC 2733
We can protect the four payload packets to their full length in one
single level with one FEC packet. This offers similar protection as
RFC 2733. The scheme is as shown in Figure 6.
+-------------------+ :
Packet A | | :
+-------------+-----+ :
Packet B | | :
+---------+---+ :
Packet C | | :
+---------+-----------------------+
Packet D | |
+---------------------------------+
:
+---------------------------------+
Packet FEC | |
+---------------------------------+
: :
:<------------- L0 -------------->:
Figure 6: FEC Scheme with Single-Level Protection
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Note: In the payload format for redundant coding as specified by
RFC 2198, the marker bit is lost as soon as the primary coding is
carried in the RED packets. So the marker bit cannot be recovered
regardless of whether or not the FEC is used.
As mentioned above, RED packet #5 will contain the FEC data (that
protects media packets A, B, C, and D) as well as the data of media
packet E. The structure of RED packet #5 is as illustrated in Figure
21.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header (RED) - 6 octets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Redundant Encoding Block Header (RED) - 4 octets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Packet Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Primary Encoding Block Header (RED) - 1 octet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Media Packet Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 21: RED Packet Structure - With FEC Data
The RTP header of the RED packets with FEC included is the same as
shown in Figure 19, with their corresponding sequence numbers and
timestamps.
In RED packet #5, the redundant encoding block header for the FEC
packet data block is as shown below in Figure 22. It will be
followed by the FEC packet data, which, in this case, includes an FEC
header (10 octets as shown in Figure 8), ULP level 0 header (4 octets
as shown in Figure 9), and the ULP level 0 data (340 octets as set
for level 0). These are followed by the primary encoding block that
contains the data of media packet E.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 1 0 1 1 0 0 0 1 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
F bit: 1 [This is the redundant coding data]
Block PT: 127 [The dynamic payload type for FEC]
TS Offset: 0 [The instance at which the FEC data is
transmitted]
Block Len: 354 [FEC header (10 octets) plus ULP level 0 header
(4 octets) and ULP level 0 data (340 octets)]
Figure 22: Redundant Encoding Block Header
11. Security Considerations
There are two ways to use FEC with encryption in secure
communications: one way is to apply the FEC on already encrypted
payloads, and the other way is to apply the FEC before the
encryption. The first case is encountered when FEC is needed by a
not trusted node during transmission after the media data is
encrypted. The second case is encountered when media data is
protected by FEC before it is transmitted through a secured
transport.
Since the protected payload of this FEC is RTP packets, applying FEC
on encrypted payloads is primarily applicable in the case of secure
RTP (SRTP) [13]. Because the FEC applies XOR across the payload, the
FEC packets should be cryptographically as secure as the original
payload. In such cases, additional encryption of the FEC packets is
not necessary.
In the following discussion, it is assumed that the FEC is applied to
the payload before the encryption. The use of FEC has implications
on the usage and changing of keys for encryption. As the FEC packets
do consist of a separate stream, there are a number of combinations
on the usage of encryption. These include:
o The FEC stream may be encrypted, while the media stream is not.
o The media stream may be encrypted, while the FEC stream is not.
o The media stream and FEC stream are both encrypted, but using
the same key.
o The media stream and FEC stream are both encrypted, but using
different keys.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
The first three of these would require all application-level
signaling protocols used to be aware of the usage of FEC, and to thus
exchange keys and negotiate encryption usage on the media and FEC
streams separately. In the final case, no such additional mechanisms
are needed. The first two cases present a layering violation, as ULP
FEC packets should be treated no differently than other RTP packets.
Encrypting just one stream may also make certain known-plaintext
attacks possible. For these reasons, applications utilizing
encryption SHOULD encrypt both streams, i.e., the last two options.
Furthermore, because the encryption may potentially be weakened by
the known relationship between the media payload and FEC data for
certain ciphers, different encryption keys MUST be used for each
stream when the media payload and the FEC data are sent in separate
streams. Note that when SRTP [13] is used for security of the RTP
sessions, different keys for each RTP session are required by the
SRTP specification.
The changing of encryption keys is another crucial issue that needs
to be addressed. Consider the case where two packets a and b are
sent along with the FEC packet that protects them. The keys used to
encrypt a and b are different, so which key should be used to decode
the FEC packet? In general, old keys need to be cached, so that when
the keys change for the media stream, the old key can be used until
it is determined that the key has changed for the ULP FEC packets as
well. Furthermore, the new key SHOULD be used to encrypt the FEC
packets that are generated from a combination of payload packets
encrypted by the old and new keys. The sender and the receiver need
to define how the encryption is performed and how the keys are used.
Altering the FEC data and packets can have a big impact on the
reconstruction operation. An attack by changing some bits in the FEC
data can have a significant effect on the calculation and the
recovery of the payload packets. For example, changing the length
recovery field can result in the recovery of a packet that is too
long. Also, the computational complexity of the recovery can easily
be affected for up to at least one order of magnitude. Depending on
the application scenario, it may be helpful to perform a sanity check
on the received payload and FEC data before performing the recovery
operation and to determine the validity of the recovered data from
the recovery operation before using them.
12. Congestion Considerations
Another issue with the use of FEC is its impact on network
congestion. In many situations, the packet loss in the network is
induced by congestions. In such scenarios, adding FEC when
encountering increasing network losses should be avoided. If it is
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RFC 5109 RTP Payload Format for Generic FEC December 2007
used on a widespread basis, this can result in increased congestion
and eventual congestion collapse. The applications may include
stronger protections while at the same time reduce the bandwidth for
the payload packets. In any event, implementations MUST NOT
substantially increase the total amount of bandwidth in use
(including the payload and the FEC) as network losses increase.
The general congestion control considerations for transporting RTP
data apply; see RTP [1] and any applicable RTP profile (e.g., RTP/AVP
[14]). An additional requirement if best-effort service is being
used is that users of this payload format MUST monitor packet loss to
ensure that the packet loss rate is within acceptable parameters.
Packet loss is considered acceptable if a TCP flow across the same
network path, and experiencing the same network conditions, would
achieve an average throughput, measured on a reasonable timescale,
that is not less than the RTP flow is achieving. This condition can
be satisfied by implementing congestion control mechanisms to adapt
the transmission rate (or the number of layers subscribed for a
layered multicast session), or by arranging for a receiver to leave
the session if the loss rate is unacceptably high.
13. IANA Considerations
Four new media subtypes have been registered with IANA, as described
in this section. This registration is done using the registration
template [3] and following RFC 3555 [4].
13.1. Registration of audio/ulpfec
Type name: audio
Subtype name: ulpfec
Required parameters:
rate: The RTP timestamp rate that is used to mark the time of
transmission of the FEC packet in a separate stream. In cases in
which it is sent as redundant data to another stream, the rate
SHALL be the same as the primary encoding it is used to protect.
When used in a separate stream, the rate SHALL be larger than 1000
Hz, to provide sufficient resolution to RTCP operations. The
selected rate MAY be any value above 1000 Hz but is RECOMMENDED to
match the rate of the media this stream protects.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Optional parameters:
onelevelonly: This specifies whether only one level of FEC protection
is used. The permissible values are 0 and 1. If 1 is signaled,
only one level of FEC protection SHALL be used in the stream. If
0 is signaled, more than one level of FEC protection MAY be used.
If omitted, it has the default value of 0.
Encoding considerations: This format is framed (see Section 4.8 in
the template document [3]) and contains binary data.
Security considerations: The same security considerations apply to
these media type registrations as to the payloads for them, as
detailed in RFC 5109.
Interoperability considerations: none
Published specification: RFC 5109
Applications that use this media type: Multimedia applications that
seek to improve resiliency to loss by sending additional data with
the media stream.
Additional information: none
Person & email address to contact for further information:
Adam Li adamli@hyervision.com
IETF Audio/Video Transport Working Group
Intended usage: COMMON
Restrictions on usage: This media, type depends on RTP framing, and
hence is only defined for transfer via RTP [1]. Transport within
other framing protocols SHALL NOT be defined as this is a robustness
mechanism for RTP.
Author:
Adam Li adamli@hyervision.com
Change controller:
IETF Audio/Video Transport Working Group delegated from the IESG.
13.2. Registration of video/ulpfec
Type name: video
Subtype name: ulpfec
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Required parameters:
rate: The RTP timestamp rate that is used to mark the time of
transmission of the FEC packet in a separate stream. In cases in
which it is sent as redundant data to another stream, the rate
SHALL be the same as the primary encoding it is used to protect.
When used in a separate stream, the rate SHALL be larger than 1000
Hz to provide sufficient resolution to RTCP operations. The
selected rate MAY be any value above 1000 Hz, but is RECOMMENDED
to match the rate of the media this stream protects.
Optional parameters:
onelevelonly: This specifies whether only one level of FEC protection
is used. The permissible values are 0 and 1. If 1 is signaled,
only one level of FEC protection SHALL be used in the stream. If
0 is signaled, more than one level of FEC protection MAY be used.
If omitted, it has the default value of 0.
Encoding considerations: This format is framed (see Section 4.8 in
the template document [3]) and contains binary data.
Security considerations: The same security considerations apply to
these media type registrations as to the payloads for them, as
detailed in RFC 5109.
Interoperability considerations: none
Published specification: RFC 5109
Applications that use this media type: Multimedia applications that
seek to improve resiliency to loss by sending additional data with
the media stream.
Additional information: none
Person & email address to contact for further information:
Adam Li adamli@hyervision.com
IETF Audio/Video Transport Working Group
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP framing, and
hence is only defined for transfer via RTP [1]. Transport within
other framing protocols SHALL NOT be defined as this is a robustness
mechanism for RTP.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Author:
Adam Li adamli@hyervision.com
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
13.3. Registration of text/ulpfec
Type name: text
Subtype name: ulpfec
Required parameters:
rate: The RTP timestamp rate that is used to mark the time of
transmission of the FEC packet in a separate stream. In cases in
which it is sent as redundant data to another stream, the rate
SHALL be the same as the primary encoding it is used to protect.
When used in a separate stream, the rate SHALL be larger than 1000
Hz to provide sufficient resolution to RTCP operations. The
selected rate MAY be any value above 1000 Hz, but is RECOMMENDED
to match the rate of the media this stream protects.
Optional parameters:
onelevelonly: This specifies whether only one level of FEC protection
is used. The permissible values are 0 and 1. If 1 is signaled,
only one level of FEC protection SHALL be used in the stream. If
0 is signaled, more than one level of FEC protection MAY be used.
If omitted, it has the default value of 0.
Encoding considerations: This format is framed (see Section 4.8 in
the template document [3]) and contains binary data.
Security considerations: The same security considerations apply to
these media type registrations as to the payloads for them, as
detailed in RFC 5109.
Interoperability considerations: none
Published specification: RFC 5109
Applications that use this media type: Multimedia applications that
seek to improve resiliency to loss by sending additional data with
the media stream.
Additional information: none
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Person & email address to contact for further information:
Adam Li adamli@hyervision.com
IETF Audio/Video Transport Working Group
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP framing, and
hence is only defined for transfer via RTP [1]. Transport within
other framing protocols SHALL NOT be defined as this is a robustness
mechanism for RTP.
Author:
Adam Li adamli@hyervision.com
Change controller:
IETF Audio/Video Transport Working Group delegated from the IESG.
13.4. Registration of application/ulpfec
Type name: application
Subtype name: ulpfec
Required parameters:
rate: The RTP timestamp rate that is used to mark the time of
transmission of the FEC packet in a separate stream. In cases in
which it is sent as redundant data to another stream, the rate
SHALL be the same as the primary encoding it is used to protect.
When used in a separate stream, the rate SHALL be larger than 1000
Hz to provide sufficient resolution to RTCP operations. The
selected rate MAY be any value above 1000 Hz, but is RECOMMENDED
to match the rate of the media this stream protects.
Optional parameters:
onelevelonly: This specifies whether only one level of FEC protection
is used. The permissible values are 0 and 1. If 1 is signaled,
only one level of FEC protection SHALL be used in the stream. If
0 is signaled, more than one level of FEC protection MAY be used.
If omitted, it has the default value of 0.
Encoding considerations: This format is framed (see Section 4.8 in
the template document [3]) and contains binary data.
Security considerations: The same security considerations apply to
these media type registrations as to the payloads for them, as
detailed in RFC 5109.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Interoperability considerations: none
Published specification: RFC 5109
Applications that use this media type: Multimedia applications that
seek to improve resiliency to loss by sending additional data with
the media stream.
Additional information: none
Person & email address to contact for further information:
Adam Li adamli@hyervision.com
IETF Audio/Video Transport Working Group
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP framing, and
hence is only defined for transfer via RTP [1]. Transport within
other framing protocols SHALL NOT be defined as this is a robustness
mechanism for RTP.
Author:
Adam Li adamli@hyervision.com
Change controller:
IETF Audio/Video Transport Working Group delegated from the IESG.
14. Multiplexing of FEC
The FEC packets can be sent to the receiver along with the protected
payload primarily in one of two ways: as a separate stream, or in the
same stream as redundant encoding. The configuration options MUST be
indicated out of band. This section also describes how this can be
accomplished using the Session Description Protocol (SDP), specified
in RFC 2327 [8].
14.1. FEC as a Separate Stream
When the FEC packets are sent in a separate stream, several pieces of
information must be conveyed:
o The address and port to which the FEC is being sent
o The payload type number for the FEC
o Which media stream the FEC is protecting
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RFC 5109 RTP Payload Format for Generic FEC December 2007
There is no static payload type assignment for FEC, so dynamic
payload type numbers MUST be used. The SSRC of the FEC stream MUST
be set to that of the protected payload stream. The association of
the FEC stream with its corresponding stream is done by line grouping
in SDP [5] with the FEC semantics [6] or other external means.
Following the principles as discussed in Section 5.2 of RFC 3550 [1],
multiplexing of the FEC stream and its associated payload stream is
usually provided by the destination transport address (network
address and port number), which is different for each RTP session.
Sending FEC together with the payload in one single RTP session and
multiplex only by SSRC or payload type precludes: (1) the use of
different network paths or network resource allocations for the
payload and the FEC protection data; (2) reception of a subset of the
media if desired, particularly for the hosts that do not understand
FEC; and (3) receiver implementations that use separate processes for
the different media. In addition, multiplexing FEC with payload data
streams will affect the timing and sequence number space of the
original payload stream, which is usually undesirable. So the FEC
stream and the payload stream SHOULD be sent through two separate RTP
session, and multiplexing them by payload type into one single RTP
session SHOULD be avoided. In addition, the FEC and the payload MUST
NOT be multiplexed by SSRC into one single RTP session since they
always have the same SSRC.
Just like any media stream, the port number and the payload type
number for the FEC stream are conveyed in their m line in the SDP.
There is no static payload type assignment for FEC, so dynamic
payload type numbers MUST be used. The binding to the number is
indicated by an rtpmap attribute. The name used in this binding is
"ulpfec". The address that the FEC stream is on is conveyed in its
corresponding c line.
The association relationship between the FEC stream and the payload
stream it protects is conveyed through media line grouping in SDP
(RFC 3388) [5] using FEC semantics (RFC 4756) [6]. The FEC stream
and the protected payload stream form an FEC group.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
The following is an example SDP for FEC application in a multicast
session:
v=0
o=adam 289083124 289083124 IN IP4 host.example.com
s=ULP FEC Seminar
t=0 0
c=IN IP4 224.2.17.12/127
a=group:FEC 1 2
a=group:FEC 3 4
m=audio 30000 RTP/AVP 0
a=mid:1
m=application 30002 RTP/AVP 100
a=rtpmap:100 ulpfec/8000
a=mid:2
m=video 30004 RTP/AVP 31
a=mid:3
m=application 30004 RTP/AVP 101
c=IN IP4 224.2.17.13/127
a=rtpmap:101 ulpfec/8000
a=mid:4
The presence of two a=group lines in this SDP indicates that there
are two FEC groups. The first FEC group, as indicated by the
"a=group:FEC 1 2" line, consists of stream 1 (an audio stream using
PCM [14]) and stream 2 (the protecting FEC stream). The FEC stream
is sent to the same multicast group and has the same Time to Live
(TTL) as the audio, but on a port number two higher. The second FEC
group, as indicated by the "a=group:FEC 3 4" line, consists of stream
3 (a video stream) and stream 4 (the protecting FEC stream). The FEC
stream is sent to a different multicast address, but has the same
port number (30004) as the payload video stream.
14.2. FEC as Redundant Encoding
When the FEC stream is being sent as a secondary codec in the
redundant encoding format, this must be signaled through SDP. To do
this, the procedures defined in RFC 2198 [7] are used to signal the
use of redundant encoding. The FEC payload type is indicated in the
same fashion as any other secondary codec. The FEC MUST protect only
the main codec, with the payload of FEC engine coming from virtual
RTP packets created from the main codec data. The virtual RTP
packets can be very easily converted from the RFC 2198 packets by
simply (1) removing all the additional headers and the redundant
coding data, and (2) replacing the payload type in the RTP header
with that of the primary codec.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
Note: In the payload format for redundant coding as specified by
RFC 2198, the marker bit is lost as soon as the primary coding is
carried in the RED packets. So the marker bit cannot be recovered
regardless of whether or not the FEC is used.
Because the FEC data (including the ULP header) is sent in the same
packets as the protected payload, the FEC data is associated with the
protected payload by being bundled in the same stream.
When the FEC stream is sent as a secondary codec in the redundant
encoding format, this can be signaled through SDP. To do this, the
procedures defined in RFC 2198 [7] are used to signal the use of
redundant encoding. The FEC payload type is indicated in the same
fashion as any other secondary codec. An rtpmap attribute MUST be
used to indicate a dynamic payload type number for the FEC packets.
The FEC MUST protect only the main codec.
For example:
m=audio 12345 RTP/AVP 121 0 5 100
a=rtpmap:121 red/8000/1
a=rtpmap:100 ulpfec/8000
a=fmtp:121 0/5/100
This SDP indicates that there is a single audio stream, which can
consist of PCM (media format 0), DVI (media format 5), the redundant
encodings (indicated by media format 121, which is bound to red
through the rtpmap attribute), or FEC (media format 100, which is
bound to ulpfec through the rtpmap attribute). Although the FEC
format is specified as a possible coding for this stream, the FEC
MUST NOT be sent by itself for this stream. Its presence in the m
line is required only because non-primary codecs must be listed here
according to RFC 2198. The fmtp attribute indicates that the
redundant encodings format can be used, with DVI as a secondary
coding and FEC as a tertiary encoding.
14.3. Offer / Answer Consideration
Some considerations are needed when SDP is used for offer / answer
[15] exchange.
The "onelevelonly" parameter is declarative. For streams declared as
sendonly, the value indicates whether only one level of FEC will be
sent. For streams declared as recvonly or sendrecv, the value
indicates what the receiver accepts to receive.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
When the FEC is sent as a separate stream and signaled through media
line grouping in SDP (RFC 3388) [5] using FEC semantics (RFC 4756)
[6], the offering side MUST implement both RFC 3388 and RFC 4756.
The rules for offer / answer in RFC 3388 and RFC 4756 SHALL be
followed with the below additional consideration. For all offers
with FEC, the answerer MAY refuse the separate FEC session by setting
the port to 0, and remove the "a=group" attribute that groups that
FEC session with the RTP session being protected. If the answerer
accepts the usage of FEC, the answerer simply accepts the FEC RTP
session and the grouping in the offer by including the same grouping
in the answer. Note that the rejection of the FEC RTP session does
not prevent the media sessions from being accepted and used without
FEC.
When the FEC stream is sent as a secondary codec in the redundant
encoding format (RFC 2198) [7], the offering side can indicate the
FEC stream as specified in Section 14.2. The answerer MAY reject the
FEC stream by removing the payload type for the FEC stream. To
accept the usage of FEC, the answerer must in the answer include the
FEC payload type. Note that in cases in which the redundancy payload
format [7] is used with FEC as the only secondary codec, when the FEC
stream is rejected the redundant encoding payload type SHOULD also be
removed.
15. Application Statement
This document describes a generic protocol for Forward Error
Correction supporting a wide range of short block parity FEC
algorithms, such as simple and interleaved parity codes. The scheme
is limited to interleaving parity codes over a distance of 48
packets. This FEC algorithm is fully compatible with hosts that are
not FEC-capable. Since the media payload is not altered and the
protection is sent as additional information, the receivers that are
unaware of the generic FEC as specified in this document can simply
ignore the additional FEC information and process the main media
payload. This interoperability is particularly important for
compatibility with existing hosts, and also in the scenario where
many different hosts need to communicate with each other at the same
time, such as during multicast.
The generic FEC algorithm specified in this document is also a
generic protection algorithm with the following features: (1) it is
independent of the nature of the media being protected, whether that
media is audio, video, or otherwise; (2) it is flexible enough to
support a wide variety of FEC mechanisms and settings; (3) it is
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RFC 5109 RTP Payload Format for Generic FEC December 2007
designed for adaptivity, so that the FEC parameters can be modified
easily without resorting to out-of-band signaling; and (4) it
supports a number of different mechanisms for transporting the FEC
packets.
The FEC specified here also provides the user with Unequal Error
Protection capabilities. Some other algorithms may also provide the
Unequal Error Protection capabilities through other means. For
example, an Unequal Erasure Protection (UXP) scheme has been proposed
in the AVT Working Group in "An RTP Payload Format for
Erasure-Resilient Transmission of Progressive Multimedia Streams".
The UXP scheme applies unequal error protection to the media payloads
by interleaving the payload stream to be protected with the
additional redundancy information obtained using Reed-Solomon
operations.
By altering the structure of the protected media payload, the UXP
scheme sacrifices the backward compatibility with terminals that do
not support UXP. This makes it more difficult to apply UXP when
backward compatibility is desired. In the case of ULP, however, the
media payload remains unaltered and can always be used by the
terminals. The extra protection can simply be ignored if the
receiving terminals do not support ULP.
At the same time, also because the structure of the media payload is
altered in UXP, UXP offers the unique ability to change packet size
independent of the original media payload structure and protection
applied, and is only subject to the protocol overhead constraint.
This property is useful in scenarios when altering the packet size of
the media at transport level is desired.
Because of the interleaving used in UXP, delays will be introduced at
both the encoding and decoding sides. For UXP, all data within a
transmission block need to arrive before encoding can begin, and a
reasonable number of packets must be received before a transmission
block can be decoded. The ULP scheme introduces little delay at the
encoding side. On the decoding side, correctly received packets can
be delivered immediately. Delay is only introduced in ULP when
packet losses occur.
Because UXP is an interleaved scheme, the unrecoverable errors
occurring in data protected by UXP usually result in a number of
corrupted holes in the payload stream. In ULP, on the other hand,
the unrecoverable errors due to packet loss in the bitstream usually
appear as contiguous missing pieces at the end of the packets.
Depending on the encoding of the media payload stream, many
applications may find it easier to parse and extract data from a
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RFC 5109 RTP Payload Format for Generic FEC December 2007
packet with only a contiguous piece missing at the end than a packet
with multiple corrupted holes, especially when the holes are not
coincident with the independently decodable fragment boundaries.
The exclusive-or (XOR) parity check operation used by ULP is simpler
and faster than the more complex operations required by Reed-Solomon
codes. This makes ULP more suitable for applications where
computational cost is a constraint.
As discussed above, both the ULP and the UXP schemes apply unequal
error protection to the RTP media stream, but each uses a different
technique. Both schemes have their own unique characteristics, and
each can be applied to scenarios with different requirements.
16. Acknowledgments
The following authors have made significant contributions to this
document: Adam H. Li, Fang Liu, John D. Villasenor, Dong-Seek Park,
Jeong-Hoon Park, Yung-Lyul Lee, Jonathan D. Rosenberg, and Henning
Schulzrinne. The authors would also like to acknowledge the
suggestions from many people, particularly Stephen Casner, Jay
Fahlen, Cullen Jennings, Colin Perkins, Tao Tian, Matthieu Tisserand,
Jeffery Tseng, Mark Watson, Stephen Wenger, and Magnus Westerlund.
17. References17.1. Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[4] Casner, S., "Media Type Registration of RTP Payload Formats",
RFC 4855, February 2007.
[5] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
[6] Li, A., "Forward Error Correction Grouping Semantics in Session
Description Protocol", RFC 4756, November 2006.
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RFC 5109 RTP Payload Format for Generic FEC December 2007
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